1. Field of the Invention
The present invention relates to processes and arrangements for the separation of pyrite from coal in fluidized beds and, in particular, gas or air-fluidized beds.
2. Description of the Background
The use of mechanical cleaning apparatuses and arrangements for reducing the ash content of coal is well-known. Coal cleaned by such apparatuses and arrangements has an increased heating value and decreased shipping costs. The coal product of such cleaning is also more uniform in quality.
Recently, additional emphasis has been placed on removing other impurities found in coal. Particular interest has centered on removing impurities, so as to lower and control the SO.sub.2 quantity of the coal. To achieve this goal, mechanical cleaning apparatuses and arrangements have also been utilized for removing pyrite from the coal, thereby providing a "clean" coal having a reduced pyrite content. Some of these apparatuses and arrangements have proven so efficient that they are capable of reducing the sulfur content of coal to values that are so low that they permit the coal to be burned directly, without the necessity of furnishing additional apparatuses or processes for sulfur control.
The majority of mechanical cleaning devices presently in use utilize a "wet" cleaning process wherein crushed coal is first slurried with water. Then, by one of several possible methods, the high density pyrite and ash are separated from the product coal. The "clean" coal must then be de-watered prior to shipment or combustion.
Mechanical apparatuses that utilize air instead of water as the fluidizing material are also available. However, "dry" or pneumatic cleaning accounts for only a small fraction of installed coal cleaning capacity.
There are definite advantages to pneumatic cleaning over wet cleaning. Of all the processes, those that utilize pneumatic cleaning are the most acceptable from the standpoint of delivered BTU cost. Pneumatic cleaning does not contribute to water pollution, as do many wet cleaning techniques. In addition, air washed coal is much less susceptible to freezing during shipment and storage and it flows more freely in hoppers and bins. Finally, wet cleaning methods are not effective for the cleaning of extremely fine coal and pyrite particles, where surface phenomena interfere with the separation process.
It has further been disclosed to use air-fired fluidized bed principles in order to obtain the separation of denser particles from less dense particles.
In U.S. Pat. No. 4,506,608 issued to Strohmeyer an unfired type of fluidized bed is used for separating denser/larger particles from less dense/smaller particles. In this device, under steady-state conditions, the denser/larger particles settle to the lower portions of the bed, where they are removed, and the less dense/smaller particles rise to the upper portion of the bed where they are removed. Unsaturated preheated air/gas is passed through the bed to fluidize it. Feedstock solid materials are added to the bed at an intermediate location thereof.
U.S. Pat. No. 4,449,483 issued to Strohmever discloses a bed formed of a mixture of solid fuel and waste inert material particles that is fluidized by heated gas/air under steady-state conditions. When in the fluidized state, the lighter solid fuel particles separate from the heavier inert material particles. The inert materials are driven by the gas/air along the bed surface and solid fuel particles rise above the surface, each travelling to different removal points along the bed.
U.S. Pat. No. 4,576,102 issued to Rasmussen, et al. disclose the removal of tramp material from gently sloped, skewed or serpentine fluidized bed vessels. A shallow bed is fluidized. Fluidizing air and gravity gently walk the tramp material toward a disposal point.
Finally, it has also been disclosed to use air fluidized bed principles to clean pyrite from coal while utilizing the principles of particle separation to improve stratification of the material in the bed. In U.S. Pat. No. 3,774,759 issued to Weintraub, et al., a method is disclosed for the separation of particulate solids of varying densities in a fluidized bed. In this method, it is disclosed to use an air-fluidized bed with magnetite as the bed material to separate pyrite from the coal under steady-state conditions. As taught therein, the coal and the magnetite are fed together into the bed vessel. Further, it is taught that when this process is made continuous, the fluidizing bed vessel should be equipped with an inclined fluidized bed that moves towards an end wall. The end wall, in turn, has adjustable orifices formed therein that permit the stratified heavy fraction and the intermixed lighter fraction to be removed separately.
While being useful for its purposes, the process and the arrangement disclosed in Weintraub nonetheless suffers from three primary drawbacks. The first of these drawbacks is that, under the steady-state conditions taught therein, stratification of the bed material is as not complete as would be hoped, thereby making removal and recovery of the desired coal particles difficult to satisfactorily achieve. The second of these drawbacks is that the maintenance of the steady-state conditions in the bed demand a high energy input and requires a large fluidized bed vessel. Finally, the third of these drawbacks is that the apparatus for and the method of removing the stratified layers from one another is not as efficient as one would desire.
Thus, it can be seen that there remains a need for processes and arrangements for separating pyrite from coal which more efficiently utilize the principle of separation of particles on the basis of size/density in order to improve the stratification of the material in the bed, so as to facilitate the removal of "clean" coal--that is to say, coal having a reduced pyrite content--from the bed. There further remains a need for such processes and arrangements that will result in sufficiently low capital and operating costs to be economically viable.